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MATERIAL HANDLING—TRANSPORTATION OF SOLIDS         99
Bucket elevators consist of steel or malleable-iron buckets attached to endless chains or belts running about end sprockets or pulleys situated in planes of different elevation. Buckets may be attached in continuous succession (continuous bucket elevators) or, as is more generally the case, at equal spacings (standard bucket elevators). Standard bucket elevators rise in vertical planes or may be slightly inclined, while continuous bucket elevators must be installed at a slight inclination.
Vertical elevators to be self discharging must be of the double-chain type with the return run of the chains snubbed under the head sprockets, or else operated at a speed of from 200 to 960 ft. per minute in order that the discharged material may be thrown over the descending buckets and so avoid undue spill. Continuous bucket elevators and standard elevators without snub pulleys are inclined at an angle with the vertical to avoid the spill and secure correct material discharge.
Buckets for continuous elevators are V-shaped in cross-section and carry their maximum load when the inclination of the elevator is such that the front and rear of the buckets are inclined equally to the vertical. Single-chain and belt elevators usually are operated at about double the speed of continuous bucket elevators so their inclination does not have to be so pronounced. Double-chain elevators when run in a vertical plane have their buckets suspended between the chains, to permit the required snubbing, but inclined elevators of the standard type, when inclined, have their buckets attached to the chains on their backs. Chains should be of the short-pitch variety to avoid pulsations as they pass about the end sprockets and when handling abrasive material likely to raise dust they should be bushed. Supporting sprockets and rollers are used on inclined elevators to prevent the loaded side sinking and interfering with the return run of the elevator.
Elevator boots of cast iron, sheet steel or wood with steel lining and furnished with adjustable take-ups for the boot sprockets or belt pulley are employed with standard bucket elevators. The load is picked up by the buckets as they pass under the sprockets or pulleys in the boots, so the feed to the boot should be at the same rate as that at which the buckets withdraw the material, so as to avoid choking the elevator and flooding the boot. Continuous bucket elevators with their comparative slow speeds permit the load being discharged directly into the buckets.
Bucket-elevator drives, through chains or gears, should be at the head end of the elevators so that the tension side of the elevator should also be the loaded side.
Economic speeds for standard bucket elevators handling various materials are given in Table 11. These speeds are dependent upon the spacing of the buckets being 12, 15, or 18 in., depending upon the size of the buckets. For continuous bucket elevators, the speeds are about one-half those given.
TABLE 11.—ECONOMIC SPEEDS FOB BUCKET ELEVATORS Standard Type
MATERIAL Coke	SPEED, FEET PEE MINUTE      MATERIAL 100      Ore (average)	SPEED, FEET PER, MINUTE 175
Broken stone (coarse)	125      Crushed stone	175
Lump coal	.   . .      125      Sand and gravel.	175
Ashes ...      .           .	....      150     Fine coal. ...       .....	......        200
Lime and cement .........	.....       150	
The carrying (elevating) capacity of bucket elevators depends upon the size of buckets, their spacing, speed of elevator and weight of material handled.    When